Apple distributes beta versions of iOS firmware in DMG disk images instead IPSW files. This can surprise quite a few Windows users, because they need IPSW files to upgrade their devices, and Windows does not register the DMG format.
Luckily, the solution is really simple and you do not need to convert anything. The IPSW file is found inside the DMG disk image, so you just need to extract it. This can be done in several programs for Windows, like for example the popular 7-zip file archiver.Open DMG file Open IPSW file